Solar Thermal Energy

Solar thermal is a renewable energy used to heat air or water for residential and commercial buildings or process heat applications using solar energy. It's generally the most cost-effective out of all the solar technologies and addresses the largest usage of building energy in heating climates like Canada, which is space and domestic water heating.

Passive flow systems require no fans, pumps, or moving parts of any kind to function. As hot fluid naturally rises, this design relies solely on natural convection currents to circulate heat through the heater and back again very slowly (see figure 1.a). Though this design is the easiest and most affordable to build and install, dependence on slow moving convection currents to circulate fluid usually limits these system to small heating loads. None the less, passive solar heaters make for a simple, effective method for small space and water heating application.

Active flow systems are more versatile than passive systems because they incorporate fans and/or pumps, thermostats, dampers, valves and sometimes backup heaters to control, manage and direct the heat flow wherever it's needed most (see figure 1.b). These systems are more expensive to build, but have demonstrated to perform far better than their passive counterparts by delivering more heat per square foot of collector surface, making them ideal for larger heating demands.

Fig. 1.a Passive airflow

Fig. 1.b Active airflow

Solar Thermal PanelsA solar thermal panel is a solar energy technology designed to capture and redistribute solar energy for useful heating applications. Solar thermal panels work on the simple principle that dark objects absorb light and heat energy; energy from the sun is absorbed by a dark collector, which can then be utilized for a desired application by transferring it to a fluid such as air or water, as it passes through the collector area and into a living space or storage unit.

The designed operating temperature range for most solar thermal systems is typically 27-60­­­­°C+ (80-140°F), for an efficient heat transfer with minimal losses through the glazing and ductwork/piping. Solar thermal is currently the most efficient solar technology available, with documented heat transfer efficiency ratings of 60-90%+ in outside temperatures lower than -30°C/-22°F. Because of this and the fact that the sun is a free, abundant, renewable fuel source - solar thermal systems generally have a COP (coefficient of power) of 60+, which means that a system will generate over 60 times more energy than what it requires to operate. Solar thermal can also be coupled with other heating technologies where applicable, and set up to work together through a system of thermostat controls to optimize conventional energy cost savings; a typical hybrid solar heating system would be configured so that when the sun is shining and there's a need for heat, the primary heat source (electric, oil/gas, etc.) will shut off and the solar thermal system will activate - when the airflow in the solar thermal system drops below a specific temperature and can no longer substitute heating requirements, then the primary heating source automatically turns on and takes over again.

Direct Use SystemsThere is often less heat loss with a direct use system (see figures 1.a & 1.b), and they can be a big advantage in spaces that require heat only during the day. Zone heating is the most successful way to utilize direct solar heating. Rather than trying to heat an entire house or office with an undersized collector, you are blowing the solar heat into the two or three rooms that need it the most. Controls and dampers for these simple installations are straightforward and fairly inexpensive, and payback will be rapid.

Multi-mode SystemsMulti-mode systems (figure 2.b) deliver heat to two or more different points of use and therefore require more electrical controls than do the direct or crawl-space systems. This added complexity and expense is usually only justified if the collector is large (more than 15 to 20 percent of the heated floor area) and delivers more heat than is needed in one part of the house. Buildings with central, forced-air furnace systems are a logical choice for retrofitting a multi-mode solar heating system, since the ductwork is already run to all parts of the house and it can be used for distributing both the solar and the back-up heat (conventional wood, oil, or gas furnace).

Thermal StorageA system with a separate heat storage component has definite advantages over the simpler systems. Having storage means that a larger collector can be used because any surplus heat can be stored for night time use rather than wasted in daytime overheating of the living space (see figure 2.a). Storage systems add convenience and efficiency in large systems, but not without significant added cost for a rock storage bin and necessary additional controls (dampers, etc.). In fact, this cost can equal the cost of the collector itself. The first thing to consider when planning for storage is whether or not the size of the collector justifies it. Building a separate heat storage area isn't usually cost-effective unless your collector is larger than 20 percent of the floor area to be heated. The collector itself should also be larger than 200 square feet. Smaller collectors find their best use in heating a crawl space or in direct use systems.

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